EIGRP to OSPF migration

[B.SK]

Hi All , I am currently working on a project where I need to migrate the routing protocol from EIGRP to OSPF in one of our company data center. There are around 100 network devices in the site which include both Cisco and Brocade  , all the access layer switches and couple of core layer devices are brocade . the remaining are all cisco (Nexus 7K, catalyst 4500, and etc).

I am expecting the recommendation on how to plan the migration and the things I need to focus in the process of executing this project.

[Jon Marshall]

Depending on the resources on the devices you should be able to run both routing protocols at the same time and as EIGRP is AD 90 and OSPF is AD 110 the EIGRP routes will stilll be preferred. 

That way you can check that you have all the routes in OSPF before turning off EIGRP. 

Couple of things to be aware of. 

Firstly OSPF can be more demanading on CPU and memory so if you have any devices that are already using a lot of resources you may want to keep an eye on those. 

Secondly, bear in mind that EIGRP external routes have an AD of 170 so if you are redistributing into EIGRP you will also need to take this into account as the OSPF routes will have a better AD. 

Jon

[B.SK]

Thanks for the inputs Jon

[sbhadrav@cisco.com]

In migration  implement  route filtering, Then it's not going to allow the EIGRP routes advertised to OSPF, back into EIGRP. At least, After that how you have it configured anyway.
If that's the case, then you could run OSPF on top of EIGRP, and the EIGRP routes will be preferred over the OSPF routes since they have an internal cost of 90 compard to OSPF's 110. Remember you will have two routes to (192.168.1.0/24), one OSPF and one EIGRP.

So you can safely run both at the same time, this will allow for an easy cut-over to OSPF.

Now if you go this route, then, you will still be redistributing OSPF into EIGRP, so you will have the other OSPF routes as External EIGRP routes with an Administrative Distance of 170. So you will have not only the internal EIGRP routes, and external EIGRP routes that are being currently redistributed, but also, the OSPF routes, since you will have OSPF runing as well. So in this case, the routes you are currently redistributing into EIGRP from OSPF, will be less preferred than te OSPF routes due to same prefix and AD of 110.
It would lower the AD of all external EIGRP routes to below 110 so they will be preferred untill the cut-over.
You would need to do this in logical steps as well, to prevent any issues.

[paul driver]

Hello

Possible a couple of caveats to this-

As noted by Jon - The administrative distance for external prefixes is higher in eigrp then ospf, As such you will need to increase the administrative distance of the ospf process to be higher than 170 before populating ospf.

Router ospf 1
distance 175

However when you ready to migrate over to ospf, DONT at that time remove the eigrp process but just decrease the ospf administrative distance to be lower than any eigrp internal routes (AD 90)

Router ospf 1
distance 75

This way if you need to back out you will only need to increase the opsf administrative distance once gain.

As for eigrp summary routes (AD 5) I guess you may need to manually remove these after you have advertised them in OSPF

[chrihussey]

Everyone's comments are spot on.

Migrated from EIGRP to OSPF on a large network about a year ago. Implemented OSPF parallel to EIGRP with the distance adjustments. Started at the perimeter and worked back to the core. It was during a maintenance window, but I don't think anyone would have really noticed the work was being done. It's all in the planning.

Regards